The present invention relates to a printing mechanism of a serial printer which can use any one of plural kinds of print thimbles. More particularly, this invention relates to a system for recognizing the kind of the print thimble interchangeably mounted on the serial printer.
In the serial printer of this type, the print thimbles can be interchanged. Print thimbles having characters different in size, type and number are prepared, and the user selects one of them according to the desired size, type and number of characters to be printed and mounts it on the printer.
The print thimble has a plurality of fingers that are formed like a petal or a wheel. A plurality of characters are arranged on the outer circumferential surface of the fingers. Arrangement, size and number of characters are different depending upon the kind of a print thimble. Therefore, for each character to be printed, the printer is required to change the impact position, hammer impression force and spacing amount of the carriage in the lateral directions, depending upon the kind of a print thimble which is mounted on the printer. It is therefore necessary for the printer to recognize the kind of the print thimble mounted thereon.
A variety of systems have been proposed to recognize the kind of print thimbles mounted on the printer. One example is disclosed in U.S. Pat. No. 4,074,798, wherein coded information identifying the kind of print wheel is formed at a predetermined position on the print wheel and is read by an optical sensor. Another example is disclosed in Japanese Patent Disclosure No. 98376/1982, wherein a magnetic film is stuck on a spoke at a position that varies depending upon the kind of print wheel. The magnetic film is detected by a magnetic sensor to recognize the kind of the print wheel.
However, in these recognition systems, the extra step of attaching optically recognizable coded information or a magnetic film to the print wheel is required in manufacturing steps of the print thimble, with the result that the manufacturing cost increases. Furthermore, because an optical sensor or a magnetic sensor is used to recognize the kind of a print thimble, the output thereof is disturbed by the dust adhering to the print wheel or by an external magnetic field making reliable recognition difficult. Further, a circuit for recognizing the output of the sensor is expensive.